Fuel Cell System

ABSTRACT

There is disclosed a fuel cell system capable of stably operating auxiliary devices driven at a high voltage and the like, even in a case where a poisoned electrode catalyst is recovered or a fuel cell is warmed up. On detecting that the electrode catalyst is poisoned, a controller derives a target operation point which is sufficient for recovering an activity of the poisoned electrode catalyst. Then, shift of the operation point from a usual operation point to a low-efficiency operation point is realized so that an output power is held to be constant.

TECHNICAL FIELD

The present invention relates to a fuel cell system.

BACKGROUND ART

In general, a fuel cell has a poor actuation property as compared withanother power source. A power generation efficiency of such a fuel celldecreases owing to lowering of a temperature and poisoning of anelectrode catalyst, and there occurs a case where a desiredvoltage/current cannot be supplied so that an apparatus (a motor or thelike) cannot be actuated.

In view of such a situation, a method is suggested in which at least oneof an anode fuel (e.g., a fuel gas) and a cathode fuel (e.g., anoxidizing gas) to be supplied to electrodes is brought into a shortagestate, and an overvoltage of a part of the electrodes is increased toraise the temperature of the fuel cell, whereby the poisoned electrodecatalyst is recovered and the fuel cell is warmed up (e.g., see PatentDocument 1 described below).

[Patent Document 1] Japanese Patent Publication No. 2003-504807

DISCLOSURE OF THE INVENTION

However, in a case where a poisoned electrode catalyst is recovered anda fuel cell is warmed up by the above method, there are problems that avoltage of the fuel cell lowers in accordance with such an operation andthat an auxiliary device (a motor of a pump or the like) to be drivenwith a high voltage cannot stably be operated.

The present invention has been developed in view of the above-mentionedsituation, and an object thereof is to provide a fuel cell systemcapable of stably operating auxiliary devices to be driven with a highvoltage, even in a case where a poisoned electrode catalyst is recoveredand a fuel cell is warmed up.

To solve the above-mentioned problem, a fuel cell system according tothe present invention is characterized by comprising: a fuel cell; aload which operates owing to a power of the fuel cell; a first voltageconversion device which is provided between the fuel cell and the loadand which converts an output of the fuel cell into a voltage to supplythe voltage to the load; operation control means for operating the fuelcell at a low-efficiency operation point having a power loss larger thanthat of a usual operation point in a case where predetermined conditionsare satisfied; and voltage conversion control means for controlling avoltage converting operation of the first voltage conversion devicebased on the operation point of the fuel cell and a driving voltage ofthe load.

According to such a constitution, even in a case where the fuel cell isoperated at the low-efficiency operation point to recover a poisonedelectrode catalyst and warm up the fuel cell, the voltage convertingoperation of the voltage conversion device is controlled based on theoperation point of the fuel cell and the driving voltage of the load.Therefore, the load can constantly stably be operated regardless of theoperation point of the fuel cell.

Here, in the above constitution, a configuration is preferable in whichthe first voltage conversion device is a booster converter to raise aterminal voltage of the fuel cell, and the voltage conversion controlmeans allows the booster converter to raise the terminal voltage of thefuel cell corresponding to the operation point to at least the drivingvoltage of the load.

Moreover, a configuration is preferable in which when a warm-upoperation of the fuel cell is required or an operation of recovering acatalyst activity of the fuel cell is required, the fuel cell isoperated at the low-efficiency operation point.

Furthermore, a configuration is preferable which further comprisesbypass means for bypassing the booster converter to supply an outputcurrent of the fuel cell to the load, while the fuel cell operates atthe usual operation point. Here, the bypass means may be a diode inwhich an anode is connected to an input side of the booster converterand in which a cathode is connected to an output side of the boosterconverter.

Moreover, in the above constitution, a configuration is preferable whichfurther comprises a chargeable/dischargeable power accumulation device;and a second voltage conversion device which converts the voltagebetween the power accumulation device and the load. Here, the secondvoltage conversion device is preferably a booster converter which raisesa discharge voltage of the power accumulation device, or a converterwhich raises or lowers the discharge voltage.

As described above, according to the present invention, even in a casewhere a poisoned electrode catalyst is recovered and a fuel cell iswarmed up, auxiliary devices to be driven with a high voltage can stablybe operated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a constitution of a main part of a fuel cellsystem according to a first embodiment;

FIG. 2A is a diagram showing a relation between an output power and apower loss according to the embodiment;

FIG. 2B is a diagram showing a relation between the output power and thepower loss according to the embodiment;

FIG. 3 is a diagram showing changes of the output power according to theembodiment;

FIG. 4 is a flow chart showing shift processing of an operation pointaccording to the embodiment;

FIG. 5A is a diagram showing changes of the output power according tothe embodiment;

FIG. 5B is a diagram showing changes of the output power according tothe embodiment;

FIG. 6 is a diagram showing a constitution of a main part of a fuel cellsystem according to a second embodiment;

FIG. 7 is a diagram showing a constitution of a main part of a fuel cellsystem according to a third embodiment;

FIG. 8 is a diagram showing a constitution of a main part of a fuel cellsystem according to a fourth embodiment;

FIG. 9 is a diagram showing a constitution of a main part of a fuel cellsystem according to a fifth embodiment;

FIG. 10 is a diagram showing changes of an FC output voltage duringstarting of the system according to the embodiment;

FIG. 11 is a diagram showing changes of the FC output voltage during thestarting of the system according to the embodiment; and

FIG. 12 is a diagram showing a rotation number of an air compressorduring the starting of the system according to the embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments according to the present invention will hereinafter bedescribed with reference to the drawings.

A. First Embodiment

FIG. 1 is a diagram showing a constitution of a main part of a fuel cellsystem 100 according to a first embodiment. The fuel cell system 100 isa vehicle system on which a fuel cell 40 is mounted as a power source,and is characterized in that a booster converter 50 is connected betweenan output end of the fuel cell 40 and a load. It is to be noted that inthe present embodiment, a fuel cell system is assumed which is to bemounted on a fuel cell hybrid vehicle (FCHV), but the system isapplicable to not only vehicles such as an electric car and a hybrid carbut also various mobile bodies (e.g., a ship, an airplane, a robot,etc.) and a stationary power source.

An oxidizing gas supply source 20 is constituted of, for example, an aircompressor, a motor which drives the air compressor, an inverter and thelike, and a rotation number of the motor and the like are adjusted toadjust an amount of an oxidizing gas to be supplied to the fuel cell 40.

A fuel gas supply source 30 is constituted of, for example, a hydrogentank, various valves and the like, and valve open degrees, ON/OFF timeand the like are adjusted to control an amount of a fuel gas to besupplied to the fuel cell 40.

The fuel cell 40 is means for generating a power from a reactive gas(the fuel gas and the oxidizing gas) to be supplied, and a fuel cell ofany type such as a solid polymer type, a phosphate type or a dissolvingcarbonate type can be used. The fuel cell 40 has a stack structure inwhich a plurality of unitary cells including an MEA and the like arelaminated in series. A terminal voltage (hereinafter referred to as theFC voltage) and an output current (hereinafter referred to as the FCcurrent) of this fuel cell 40 are detected by a voltage sensor 110 and acurrent sensor 120, respectively. A fuel gas such as a hydrogen gas issupplied from the fuel gas supply source 30 to a fuel pole (an anode) ofthe fuel cell 40, whereas an oxidizing gas such as air is supplied fromthe oxidizing gas supply source 20 to an oxygen pole (a cathode). The FCvoltage of this fuel cell 40 is supplied to the booster converter 50 viaconnection relays R1, R2. The connection relays R1, R2 are controlled toturn ON/OFF based on a switch signal to be supplied from a controller40.

The booster converter (a first voltage conversion device) 50 raises theFC voltage supplied from the fuel cell 40 to a system required voltage(a driving voltage of the load) under control of the controller 10, tosupply the voltage to inverters 60, 80. It is to be noted that in thefollowing description, the voltage input into the booster converter 50before raised will be referred to as an input voltage Vin, and theraised voltage output from the booster converter 50 will be referred toas an output voltage Vout.

This booster converter 50 includes a reactor L1, a diode D1 forrectification, and a switching element SW1 including an IGBT and thelike. One end of the reactor L1 is connected to the connection relay R1,and the other end thereof is connected to a collector of the switchingelement SW1. The switching element SW1 is connected between power linesand earth lines of the inverters 60, 80. Specifically, the collector ofthe switching element SW1 is connected to the power lines, and anemitter thereof is connected to the earth lines. In such a constitution,when the switch SW1 is first turned ON, a current flows to the fuel cell40→the inductor L1→the switch SW1. At this time the reactor L1 isexcited with a direct current to accumulate magnetic energy.

Subsequently, when the switch SW1 is turned OFF, an induced voltage dueto the magnetic energy accumulated in the inductor L1 is superimposed onthe FC voltage (the input voltage Vin) of the fuel cell 40 to output anoperation voltage (the output voltage Vout) higher than the inputvoltage Vin from the inductor L1, and the output current is output viathe diode D1. The controller 10 appropriately changes an ON/OFF duty(described later) of this switch SW1 to obtain a desired output voltageVout.

Each of the inverters 60, 80 is, for example, a PWM inverter of a pulsewidth modulation system, and converts a direct-current power suppliedfrom the booster converter 50 into a three-phase alternate-current powerbased on a control command given from the controller 80, to supply thepower to motors 70, 90.

In more detail, the inverter 60 converts, into the three-phasealternate-current power, the direct-current power supplied from thebooster converter 50 via a capacitor C1, to supply the power to thetraction motor 70. The traction motor 70 is a motor (i.e., a powersource of a mobile body) to drive wheels 75L, 75R, and the rotationnumber of such a motor is controlled by the inverter 60. It is to benoted that the capacitor C1 smoothens the direct-current voltagesupplied from the booster converter 50 to supply the voltage to theinverter 60.

On the other hand, the inverter 80 converts the direct-current voltagesupplied from the booster converter 50 into the three-phasealternate-current power to supply the power to auxiliary devices 90. Theauxiliary devices 90 are constituted of a vehicle auxiliary device, anFC auxiliary device and the like. It is to be noted that the vehicleauxiliary device is any type of power device for use in operating avehicle and the like (a light, an air conditioner, a hydraulic pump,etc.), and the FC auxiliary device is any type of power device for usein operating the fuel cell 40 (pumps for supplying the fuel gas and theoxidizing gas, etc.).

The controller (operation control means, voltage conversion controlmeans) 10 is constituted of a CPU, an ROM, an RAM and the like, andsystem sections are centrically controlled based on sensor signals inputfrom the voltage sensor 110 and the current sensor 120, a temperaturesensor which detects a temperature of the fuel cell 40, an acceleratorpedal sensor which detects an open degree of an accelerator pedal andthe like.

Moreover, the controller 10 detects by the following method whether ornot an electrode catalyst of the fuel cell 40 is poisoned, and performsprocessing to switch an operation point of the fuel cell 40 so as torecover characteristics of the poisoned electrode catalyst (describedlater).

A memory 160 is, for example, a rewritable nonvolatile memory in whichinitial cell characteristic data indicating cell characteristics in aninitial state (e.g., during shipping of a manufactured cell) of the fuelcell 40 and the like are stored. The initial cell characteristic data isa two-dimensional map showing a relation between a current density and avoltage of the fuel cell 40 in the initial state, and the voltage lowersas the current density increases.

As is known, when the electrode catalyst of the fuel cell 40 ispoisoned, the cell characteristics lower. With the equal voltage, thecurrent density after poisoning decreases as compared with that beforethe poisoning (the current density indicated by the initial cellcharacteristic data). In the present embodiment, the FC voltage and theFC current detected by the voltage sensor 110 and the current sensor 120are compared with the initial cell characteristic data by use of theabove-mentioned characteristics, to detect whether or not the electrodecatalyst is poisoned. More specifically, when the voltage sensor 110 andthe current sensor 120 detect the FC voltage and the FC current, thecontroller (detection means) 10 compares the detection result with thecurrent density at the equal voltage in the initial cell characteristicdata. As a result of such comparison, when the following formulas (1),(2) are established, it is judged that the electrode catalyst ispoisoned. On the other hand, when the following formulas (1), (2) arenot established, it is judged that the electrode catalyst is notpoisoned.

Vfc=Vs  (1), and

Ifc<Is+α  (2),

in which Vfc; an FC voltage,

Vs; a voltage in the initial cell characteristic data,

-   -   Ifc; an FC current,    -   Is; a current density in the initial cell characteristic data,        and    -   α; a predetermined value.

It is to be noted that in the above description, it is detected usingthe initial cell characteristic data whether or not the electrodecatalyst is poisoned, but needless to say, it may be detected by anothermethod whether or not the electrode catalyst is poisoned. For example,when the electrode catalyst is poisoned by carbon monoxide, a known COconcentration sensor is provided, and a relation between a COconcentration and a measured voltage value may beforehand be inspectedand mapped to detect, based on the detected CO concentration or thelike, whether or not the electrode catalyst is poisoned. The operationpoint of the fuel cell 40 will hereinafter be described with referenceto the drawing.

FIGS. 2A and 2B are diagrams showing a relation between an output powerand a power loss at a time when the fuel cell is operated at differentoperation points. The abscissa indicates the FC current and the ordinateindicates the FC voltage. Moreover, an open circuit voltage (OCV) shownin FIGS. 2A and 2B is a voltage in a state in which any current is notcirculated through the fuel cell.

The fuel cell 40 capable of obtaining the current and voltagecharacteristics (hereinafter referred to as the IV characteristic) shownin FIGS. 2A and 2B is usually operated at an operation point (Ifc1,Vfc1) at which the power loss is small with respect to the output power(see FIG. 2A). However, when the electrode catalyst of the fuel cell 40is poisoned, an inner temperature of the fuel cell 40 needs to be raisedto recover an activity of the electrode catalyst. Therefore, in thepresent embodiment, the operation of the fuel cell shifts to anoperation point (Ifc2, Vfc2) having a large power loss while securing anecessary output power, thereby recovering the activity of the poisonedelectrode catalyst (see FIG. 2B). Here, output powers Pfc at theoperation points shown in FIGS. 2A and 2B, a power loss Ploss, arelation between the output voltages Pfc and a relation between thepower losses Ploss are as follows.

<Concerning the Operation Point (Ifc, Vfc1)>

Ifc1*Vfc1=Pfc1  (3)

Ifc1*OCV−Pfc1=Ploss1  (4)

<Concerning the Operation Point (Ifc2, Vfc2)>

Ifc2*Vfc2=Pfc2  (5)

Ifc2*OCV−Pfc2=Ploss2  (6)

<Relations Between the Output Powers and Between the Power Losses>

Pfc1=Pfc2  (7)

Ploss1<Ploss2  (8)

FIG. 3 is a diagram showing changes of the output power at a time whenthe fuel cell is operated while the operation point is shifted. Theabscissa indicates the FC current, and the ordinate indicates the FCvoltage and the output power. It is to be noted that in FIG. 3, for thesake of convenience, the IV characteristic of the fuel cell is shownwith a straight line (hereinafter referred to as the IV line). Operationpoints (Ifc1, Vfc1), (Ifc2, Vfc2) on the IV line correspond to theoperation points (Ifc1, Vfc1), (Ifc2, Vfc2) shown in FIG. 2.

As shown in FIG. 3, with regard to the output power Pfc of the fuel cell40, as the FC voltage Vfc decreases, the output power Pfc increases atan operation point on the IV line shown on the left side of a maximumoutput operation point (Ifcmax, Vfcmax) at which a maximum output powerPfcmax is obtained. On the other hand, at an operation point on the IVline shown on the right side of the maximum output operation point, theoutput power Pfc decreases, as the FC voltage Vfc decreases.

As described above, the power loss Ploss increases, as the FC voltageVfc decreases. Therefore, even when the fuel cell 40 is operated tooutput the equal power, the power loss Ploss is large in a case wherethe fuel cell is operated at the operation point on the IV line shown onthe right side of the maximum output operation point (e.g., theoperation point (Ifc1, Vfc1)) as compared with a case where the fuelcell is operated at the operation point on the IV line shown on the leftside of the maximum output operation point (e.g., the operation point(Ifc2, Vfc2)). Therefore, in the following description, the operationpoint on the IV line at which the output power Pfc increases with thedecrease of the FC voltage Vfc is defined as a usual operation point,and the operation point on the IV line at which the output power Pfcdecreases with the decrease of the FC voltage Vfc is defined as alow-efficiency operation point. It is to be noted that the usualoperation point and the low-efficiency operation point are as follows.

<Concerning the Usual Operation Point (Ifc, Vfc)>

Ifc≦Ifcmax  (9)

Vfcmax≦Vfc  (10)

<Concerning the Low-Efficiency Operation Point (Ifc, Vfc)>

Ifcmax<Ifc  (11)

Vfc<Vfcmax  (12)

Next, the operation point shift processing to be executed by thecontroller 80 will be described with reference to FIGS. 4 and 5.

FIG. 4 is a flow chart showing the shift processing of the operationpoint, and FIG. 5 shows diagrams of changes of the output power at atime when the operation point is shifted. It is to be noted that in thefollowing description, it is assumed a case where the operation point ofthe fuel cell 40 is shifted from the usual operation point (Ifc1, Vfc1)to the low-efficiency operation point (Ifc2, Vfc2) in order to recoverthe activity of the poisoned electrode catalyst (see FIGS. 5A and 5B).In the following description, devices which consume the power outputfrom the booster converter 50, for example, the inverters 60, 80, thetraction motor 70, the auxiliary devices 90 and the like connected tothe booster converter 50 will generically be referred to as loads.

The controller 10 first judges whether or not an operation to recoverthe catalyst activity is required (step S1). Specifically, the FCvoltage and the FC current detected by the voltage sensor 110 and thecurrent sensor 120 are compared with the initial cell characteristicdata to detect whether or not the electrode catalyst is poisoned. Whenthe electrode catalyst is not poisoned, it is judged that the operationto recover the catalyst activity is not required. On the other hand,when the electrode catalyst is poisoned, it is judged that the operationto recover the catalyst activity is required.

When the electrode catalyst is not poisoned, the controller (theoperation control means) 10 continues operating at the usual operationpoint based on the driving power of the load (a system required power).More specifically, the controller (the voltage conversion control means)10 grasps the system required power, and then determines the usualoperation point (Ifc1, Vfc1) corresponding to the system required powerwith respect to a power-operation point correspondence map (not shown)stored in the memory 160 or the like, to perform the operation at thedetermined usual operation point. Here, during the operation at theusual operation point, the operation of the booster converter 50 isstopped (the switching element SW1; “OFF”), so that the input voltageVin and output voltage Vout of the booster converter 50 become equal.For example, when a system required voltage Vreq is 350 V, thecontroller 10 sets Vfc1 of the usual operation point corresponding tothe system required power to 350 V. During the operation at the usualoperation point, the operation of the booster converter 50 is stopped,so that Vfc1=Vin=Vout=350 V results.

On the other hand, when the electrode catalyst is poisoned, thecontroller 10 confirms the operation point (here, the usual operationpoint (Ifc1, Vfc1)) at the present time (step S2), then derives anadequate operation point (a target operation point) of the fuel cell 40so as to recover the activity of the poisoned electrode catalyst (stepS3). One example will be described. For example, when the fuel cell isoperated at the usual operation point (Ifc1, Vfc1) to obtain the outputpower Pfc1, the low-efficiency operation point (Ifc2, Vfc2) capable ofobtaining the output power Pfc2 (=Pfc1) equal to the above output poweris derived as the target operation point.

It is to be noted that in the poisoned electrode catalyst, a cellvoltage of the fuel cell 40 is controlled into 0.6 V or less, whereby acatalyst reducing reaction occurs to recover the catalyst activity.Therefore, the operation point which satisfies such conditions may bederived as the target operation point (details will be described later).

Then, the controller (the operation control means) 10 starts shift ofthe operation point toward the target operation point (step S4). Here,in a case where the only FC voltage is controlled to shift the operationpoint from the usual operation point (Ifc1, Vfc1) to the low-efficiencyoperation point (Ifc2, Vfc2), as shown in FIG. 5A, the output power ofthe fuel cell 40 largely fluctuates in response to the shift of theoperation point of an IV line l1 (see a power line pl1).

More specifically, in a case where the only FC voltage is controlledusing the booster converter 50 to shift the operation point, in a shiftprocess, there occurs necessity of performing a high-output operation(an operation at the maximum output operation point or the like) whichmight not be performed under a usual use environment.

To solve the problem, in the present embodiment, as shown in FIG. 5B,the FC current is controlled together with the FC voltage to realize theshift of the operation point from the usual operation point (Ifc1, Vfc1)to the low-efficiency operation point (Ifc2, Vfc2) so that the outputpower is kept constant (see a power line pl2). Specifically, the FCvoltage is lowered from Vfc1 to Vfc2 by use of the booster converter(the first voltage conversion device, a voltage conversion device) 50,and the controller (adjustment means, change means) 10 adjusts an amountof the oxidizing gas to be supplied from the oxidizing gas supply source20 (here, reduces the amount of the oxidizing gas), to control the FCcurrent.

The control of the FC voltage will be described in more detail. Forexample, when the system required voltage Vreq is 350 V, the controller10 sets Vfc2 of the low-efficiency operation point to 30 V. Then, thecontroller (the voltage conversion control means) 10 controls anoperation (the duty) of the booster converter 50 so as to satisfyVfc2=Vin=30 V while satisfying Vreq=Vout=350 V. It is to be noted thatthe duty of the booster converter 50 can be represented as follows.

Duty=(Vout−Vin)Nout  (13)

When the operation point is shifted as described above, the controller10 judges with reference to a timer (not shown) or the like whether ornot a target set time has elapsed since the operation point was shifted(step S5). Here, the target set time is time (e.g., 10 seconds) adequatefor recovering the activity of the electrode catalyst, which has elapsedsince the operation was started at the low-efficiency operation point,and can be obtained in advance by an experiment or the like. On judgingthat the target set time has not elapsed (step S5; NO), the controller10 repeatedly executes the step S5. On the other hand, on judging thatthe target set time has elapsed (step. S5; YES), the controller 10returns the shifted operation point to the operation point beforeshifted (step S6), thereby ending the processing.

As described above, according to the fuel cell system of the presentembodiment, even when the poisoned electrode catalyst is recovered, theauxiliary devices driven at the high voltage and the like can stably beoperated.

It is to be noted that as described above, with regard to the poisonedelectrode catalyst, the cell voltage of the fuel cell 40 is controlledinto 0.6 V or less to recover the catalyst activity, so that theoperation point may be derived as follows.

For example, in a case where the fuel cell 40 has a stack structure inwhich 300 cells are laminated and a system required power is 1 kW, ifthe cell voltage is set to 0.5 V (<0.6 V), the target operation point isas follows.

<Concerning the Target Operation Point (Ifc, Vfc)>

Vfc=300*0.5=150 V  (14)

Ifc=1000/150=6.7 A  (15)

Here, even in a case where the obtained target operation point is notpresent on the IV line before shifted, the FC current is controlledtogether with the FC voltage to change the IV characteristic, wherebythe obtained target operation point can be positioned on the IV line.

<Modification>

(1) In the above embodiment, the amount of the oxidizing gas to besupplied from the oxidizing gas supply source 20 is adjusted to controlthe FC current. However, an amount of a fuel gas to be supplied from afuel gas supply source 30 may be adjusted to control the FC current.

(2) In the above embodiment, in a case where it is detected that theelectrode catalyst is poisoned, the operation point of the fuel cell 40is shifted from the usual operation point to the low-efficiencyoperation point, but the operation point may be shifted at the followingtiming.

For example, a fuel cell may be operated once at a low-efficiencyoperation point during actuation of a system, and then the operationpoint may be shifted to a usual operation point to perform a systemoperation in a state in which a catalyst activity is constantly raised.When a system required power is a predetermined value or less (e.g.,around an idle output or the like), the operation point may be shiftedfrom the usual operation point to the low-efficiency operation point.Furthermore, after the system stops, the operation may be performed atthe low-efficiency operation point to recover the deteriorated catalystactivity during the operation in preparation for the next actuation.

(3) In the above embodiment, it is constituted that the operation pointof the fuel cell 40 is shifted from the usual operation point to thelow-efficiency operation point in order to recover the activity of thepoisoned electrode catalyst, but the present invention is applicable toany case that requires a warm-up operation, for example, a case wherethe warm-up operation is performed during the actuation at a lowtemperature, a case where the warm-up operation is rapidly performedbefore the stop of the system operation and the like.

One example will be described. On receiving an actuation command of thesystem from an operation switch or the like, a controller 10 detects aninner temperature of a fuel cell 40 by use of a temperature sensor (notshown). The controller (operation control means) 10 judges that thewarm-up operation is required in a case where the inner temperature ofthe fuel cell 40 is below a preset threshold temperature, and the shiftprocessing of the operation point shown in FIG. 4 is executed. Thesubsequent operation is similar to that of the present embodiment, andhence description thereof is omitted. It is to be noted that instead ofthe temperature sensor, a temperature sensor to detect an outside airtemperature, a temperature sensor to detect a temperature of arefrigerant flowing through a cooling mechanism (not shown) or the likemay be used.

B. Second Embodiment

FIG. 6 is a diagram showing a constitution of a main part of a fuel cellsystem 100 a according to a second embodiment. It is to be noted thatcomponents corresponding to those of the fuel cell system 100 shown inFIG. 1 are denoted with the same reference numerals, and detaileddescription thereof is omitted.

As shown in FIG. 6, the fuel cell system 100 a is provided with a diode(bypass means) D2 for canceling a steady loss due to a direct-currentresistance of a reactor L1. In the diode D2, an anode is connected to aprevious stage of the reactor L1 (an input side of a booster converter),whereas a cathode is connected to a subsequent stage of a diode D1 (anoutput side of the booster converter).

The diode D2 is thus provided for the following reason. That is, when anoperation of a booster converter 50 is stopped (a switching element SW1:“OFF”) during an operation at a usual operation point, an FC currentflows through the reactor L1→the diode D1, and the steady loss due tothe direct-current resistance of the reactor L1 raises a problem (seeFIG. 1). To solve the problem of the steady loss due to thisdirect-current resistance of the reactor L1, the diode D2 is provided(see FIG. 6). As a result, during the operation at the usual operationpoint, the FC current bypasses the booster converter 50 to flow throughthe diode D2→a load. On the other hand, during an operation at alow-efficiency operation point, the FC current flows through the reactorL1→the diode D1→the load, and the steady loss due to the direct-currentresistance of the reactor L1 can be canceled.

In the first and second embodiments described above, a power sourcesystem including the fuel cell 40 only has been described. However, inthird and fourth embodiments described below, a hybrid power sourcesystem including the fuel cell 40 and a battery 80 will be described.

C. Third Embodiment

FIG. 7 is a diagram showing a constitution of a main part of a fuel cellsystem 100 b according to a third embodiment. It is to be noted thatcomponents corresponding to those of the fuel cell system 100 a shown inFIG. 6 are denoted with the same reference numerals, and detaileddescription thereof is omitted.

As shown in FIG. 7, on the fuel cell system 100 a, a power source systemincluding a fuel cell 40 and a booster converter 50, and a power sourcesystem including a battery 130 and a booster converter 140 are mounted.

The battery (a power accumulation device) 130 is achargeable/dischargeable secondary cell, and constituted of, forexample, a nickel hydrogen battery or the like. Needless to say, insteadof the battery 130, a chargeable/dischargeable accumulator (e.g., acapacitor) may be provided except a secondary cell. The battery 130 isconnected to a load via the booster converter 140. However, it isassumed that a discharge voltage Vba of the battery 130 is lower than asystem required voltage Vreq. For example, in a case where the systemrequired voltage fluctuates from 300 to 350 V, the battery 130 islimited to a battery in which the discharge voltage Vba is 300 V or less(e.g., 200 to 299 V).

The booster converter (a second voltage conversion device) 140 raisesthe discharge voltage Vba supplied from the battery 130 to the systemrequired voltage (a voltage between A and B shown in FIG. 7) undercontrol of a controller 10, to supply the voltage to inverters 60, 80.For example, when the system required voltage Vreq is set to 350 V andthe discharge voltage Vba of the battery 130 is set to 250 V, thecontroller 10 controls a duty of the booster converter 140, whereby thedischarge voltage Vba (=250 V) is raised to the system required voltage(=350 V).

This booster converter 50 includes a reactor L2, and switching elementsSW2, SW3 including an IGBT and the like. One end of the reactor L1 isconnected to a power line of the battery 130, and the other end thereofis connected between an emitter of the switching element SW2 and acollector of the switching element SW3.

The switching elements SW2, SW3 are connected in series between thepower line and the earth line on an inverter side. A collector of theswitching element SW2 is connected to the power line, and an emitter ofthe switching element SW3 is connected to the earth line.

A capacitor C2 smoothens a direct-current voltage supplied from thebattery 130 to supply the voltage to the booster converter 140, whereasa capacitor C3 smoothens a direct-current voltage supplied from thebooster converter 140 to supply the voltage to the inverters 60, 80. Itis to be noted that a booster operation of the booster converter 140 issubstantially similar to that of the first embodiment, and hencedescription thereof is omitted.

According to such a constitution, auxiliary devices driven at a highvoltage and the like can stably be operated. In addition, the load canefficiently be driven using the fuel cell and the battery.

In the third embodiment described above, a case where the dischargevoltage Vba of the battery 130 constantly does not overlap with thesystem required voltage, specifically a case where the discharge voltageVba of the battery 130 is constantly below the system required voltageVreq has been described, but in the following fourth embodiment, a casewhere the discharge voltage Vba of the battery 130 overlaps with thesystem required voltage Vreq will be described.

D. Fourth Embodiment

FIG. 8 is a diagram showing a constitution of a main part of a fuel cellsystem 100 c according to a third embodiment. It is to be noted thatcomponents corresponding to those of the fuel cell system 100 b shown inFIG. 7 are denoted with the same reference numerals, and detaileddescription thereof is omitted.

As shown in FIG. 8, the fuel cell system 100 c is provided with aconverter (a second voltage conversion device) 150 instead of thebooster converter 140 (see FIG. 7).

The converter 150 raises or lowers a discharge voltage Vba supplied froma battery 130 to a system required voltage (a voltage between A and Bshown in FIG. 8) under control of a controller 10, to supply the voltageto inverters 60, 80. For example, when a system required voltage Vreq isset to 350 V and the discharge voltage Vba of the battery 130 is set to300 V, the controller 10 controls a duty of the converter 150, wherebythe discharge voltage Vba (=300 V) is raised to the system requiredvoltage (=350 V). On the other hand, when the system required voltageVreq is set to 250 V and the discharge voltage Vba of the battery 130 isset to 300 V, the controller 10 controls the duty of the converter 150,whereby the discharge voltage Vba (=300 V) is lowered to the systemrequired voltage (=250 V). The duty of the converter 150 can berepresented as follows.

Duty=Vout/(Vin+Vout)  (16)

This converter 150 is a full bridge converter including a reactor L3,and four switching elements SW4 to SW7 including an IGBT and the like.One end of the reactor L3 is connected between an emitter of theswitching element SW4 and a collector of the switching element SW5, andthe other end thereof is connected between an emitter of the switchingelement SW6 and a collector of the switching element SW7.

The switching elements SW4, SW5 are connected in series between a powerline and an earth line on a battery side. A collector of the switchingelement SW4 is connected to the power line, and an emitter of theswitching element SW5 is connected to the earth line. The switchingelements SW6, SW7 are connected in series between a power line and anearth line on an inverter side. A collector of the switching element SW6is connected to the power line, and an emitter of the switching elementSW7 is connected to the earth line.

According to such a constitution, even when the discharge voltage of thebattery overlaps with the system required voltage, a load canefficiently be driven.

<Modification>

In the third and fourth embodiments described above, the diode D2 forcanceling the steady loss due to the direct-current resistance of thereactor L1 is provided, but when the steady loss of the reactor L1 doesnot have to be considered, the diode D2 may not be provided.

Moreover, in the above-mentioned embodiments, the IGBT is illustrated asthe switching element, but the present invention is applicable to anyelement that can be switched, for example, an MOSFET, a bipolartransistor or the like.

F. Fifth Embodiment

FIG. 9 is a diagram showing a constitution of a main part of a fuel cellsystem 100 d according to a fourth embodiment. It is to be noted thatcomponents corresponding to those of the fuel cell system 100 c shown inFIG. 8 are denoted with the same reference numerals, and detaileddescription thereof is omitted.

An oxidizing gas supply source 20 is constituted of an air compressor700 and the like. The air compressor 700 adjusts an amount of anoxidizing gas to be supplied to a fuel cell 40 under control of acontroller 10.

FIGS. 10 to 12 are diagrams showing changes of an FC output voltage, anFC output power and a rotation number of the air compressor duringstarting of the system, respectively.

Prior to the system starting, the controller (inhibition means) 10 ofthe fuel cell system 100 d controls a rotation number Rfc to supply theoxidizing gas to the fuel cell 40 for a predetermined time, and thenstops the supply (see c1 shown in FIG. 12).

In accordance with this supply of the oxidizing gas, an FC outputvoltage Vmfc rises to the vicinity of an open circuit voltage (OCV) (seea1 shown in FIG. 10), but during the system starting, the FC outputvoltage Vmfc raised to the vicinity of the OCV needs to be lowered to avoltage of a low-efficiency operation region. Control for this purposewill be described. The controller (first determination means) 10 firstadds up a system required power (driving energy of auxiliary devices orthe like) Pre and a battery charging power Pba (see b1 shown in FIG. 11)to determine an FC output instruction power Pfc (see equation (17)).

Pfc=Pre+Pba  (17)

Subsequently, the controller (second determination means) 10 divides theobtained FC output instruction power Pfc by the FC output voltage (anactually measured value) Vmfc to obtain an FC output instruction currentIfc (see equation (18)).

Ifc=Pfc/Vmfc  (18)

Furthermore, the controller (third determination means) 10 obtains adeviation between the obtained FC output instruction current Ifc and theFC output current (the actually measured value) Imfc to feed back theobtained deviation to a change amount of the FC output voltage Vmfc (=adifference between the previously measured output voltage and thepresently measured output voltage), whereby an FC output instructionvoltage Vfc is calculated.

In a case where the controller 10 confirms that the FC output voltagelowers to the voltage of the low-efficiency operation region, thecontroller restarts the supply of the oxidizing gas by the aircompressor 700 to start the operation at a low-efficiency operationpoint.

As described above, according to the present embodiment, when the FCoutput voltage is lowered from the vicinity of the OCV to the voltage ofthe low-efficiency operation region, the supply of the oxidizing gas isstopped to obtain the deviation between the FC output instructioncurrent Ifc and the FC output current (the actually measured value)Imfc, followed by feeding back the obtained deviation to the changeamount of the FC output voltage Vmfc, whereby the operation at thelow-efficiency operation point can quickly be started.

It is to be noted that in the above embodiment, a case where the FCoutput voltage is lowered from the vicinity of the OCV to the voltage ofthe low-efficiency operation region has been described, but the presentinvention is applicable to any case where the FC output voltage islowered to a target voltage.

1. A fuel cell system comprising: a fuel cell; a load which operatesowing to a power of the fuel cell; a first voltage conversion devicewhich is provided between the fuel cell and the load and which convertsan output of the fuel cell into a voltage to supply the voltage to theload; an operation control device for operating the fuel cell at alow-efficiency operation point having a power loss larger than that of ausual operation point in a case where predetermined conditions aresatisfied; and a voltage conversion control device for controlling avoltage converting operation of the first voltage conversion devicebased on the operation point of the fuel cell and a driving voltage ofthe load.
 2. The fuel cell system according to claim 1, wherein thefirst voltage conversion device is a booster converter to raise aterminal voltage of the fuel cell, and the voltage conversion controldevice allows the booster converter to raise the terminal voltage of thefuel cell corresponding to the operation point to at least the drivingvoltage of the load.
 3. The fuel cell system according to claim 1,wherein when a warm-up operation of the fuel cell is required or anoperation of recovering a catalyst activity of the fuel cell isrequired, the fuel cell is operated at the low-efficiency operationpoint.
 4. The fuel cell system according to claim 1, further comprising:a bypass device for bypassing the booster converter to supply an outputcurrent of the fuel cell to the load, while the fuel cell operates atthe usual operation point.
 5. The fuel cell system according to claim 4,wherein the bypass device is a diode in which an anode is connected toan input side of the booster converter and in which a cathode isconnected to an output side of the booster converter.
 6. The fuel cellsystem according to claim 1, further comprising: achargeable/dischargeable power accumulation device; and a second voltageconversion device which converts the voltage between the poweraccumulation device and the load.
 7. The fuel cell system according toclaim 6, wherein the second voltage conversion device is a boosterconverter which raises a discharge voltage of the power accumulationdevice, or a converter which raises or lowers the discharge voltage. 8.A fuel cell system comprising: a fuel cell which generates a power byuse of a fuel gas and an oxidizing gas; a load connected to the fuelcell; a chargeable/dischargeable power accumulation device interposedbetween the fuel cell and the load; a voltage conversion device whichcontrols a terminal voltage of the fuel cell to determine operationpoints of the fuel cell and the power accumulation device; an operationcontrol device for operating the fuel cell at a low-efficiency operationpoint having a power loss larger than that of a usual operation point ina case where predetermined conditions are satisfied; and a change devicefor allowing the voltage conversion device to adjust the terminalvoltage of the fuel cell, and controlling an amount of the oxidizing gasto be supplied to the fuel cell to adjust an output current of the fuelcell, thereby changing the operation point of the fuel cell.
 9. A fuelcell system comprising: a fuel cell which generates a power by use of afuel gas and an oxidizing gas; an inhibition device for inhibitingsupply of the oxidizing gas in a case where predetermined conditions aresatisfied; a first determination device for determining a target powerof the fuel cell at a time when the supply of the oxidizing gas isinhibited; a second determination device for determining a targetcurrent based on the target power and an actually measured voltage ofthe fuel cell; and a third determination device for feeding back adeviation between the target current and an actually measured current ofthe fuel cell to a change amount of the actually measured voltage of thefuel cell to determine a target voltage value.
 10. The fuel cell systemaccording to claim 9, wherein the inhibition device stops the supply ofthe oxidizing gas in a case where an output voltage of the fuel cell islowered from the vicinity of an opened circuit voltage to a voltage at alow-efficiency operation point, and the first determination devicedetermines the target power of the fuel cell at a time when the supplyof the oxidizing gas is stopped.
 11. The fuel cell system according toclaim 1, wherein the operation control device shifts the operation pointin a state in which an output power is held to be constant.
 12. The fuelcell system according to claim 1, further comprising: achargeable/dischargeable power accumulation device; and a second voltageconversion device which converts the voltage between the poweraccumulation device and the load, wherein the voltage conversion controldevice controls a voltage converting operation of the first voltageconversion device and a voltage converting operation of the secondvoltage conversion device in accordance with the driving voltage of theload.
 13. The fuel cell system according to claim 12, wherein thevoltage conversion control means controls a voltage converting operationof the first voltage conversion device in accordance with the drivingvoltage of the load, and controls a voltage converting operation of thesecond voltage conversion device in accordance with the convertedvoltage obtained by the voltage converting operation.